Conventionally, varistors have been used to protect switchboards against surges created by lightning. When a surge current flows in, the varistor can release the current to protect a switchboard against the surge. However, the inflow of an excessive surge current will cause to the varistor to go into a short mode, resulting in possible danger of ignition. On that account, a thermal fuse is connected in series to the varistor so as to cut off the varistor, thereby preventing the ignition. Moreover, when the varistor is cut off, an indication is also made to indicate that the varistor has been cut off.
Note that Patent Literature 1, for example, is known as information on the conventional technology related to the invention of the present application.